Apparatus for producing binary crystalline compounds



April 0 E. DEYRIS 3,507,625

APPARATUS FOR PRODUCING BINARY CRYSTALLINE COMPOUNDS Filed Dec. 8, 1966INVENTOR.

EMILE DEYRIS AGEN United States Patent r 3,507,625 APPARATUS FORPRODUCING BINARY CRYSTALLINE COMPOUNDS Emile Deyris, Caen, France,assignor, by mesne assignments, to US. Philips Corporation, New York,N.Y.,

a corporation of Delaware Filed Dec. 8,,1966, Ser. No. 600,124 Claimspriority, applicastion France, Jan. 10, 1966,

2 Int. Cl. B01j 17/18; C01b 27/00 US. Cl. 23-473 4 Claims ABSTRACT OFTHE DISCLOSURE The invention relates to a method and a device forproducing binary single-crystal compounds, particularly of binarysemiconductor single crystals.

It is known to produce not only the generally em- 'ployed semiconductorbodies of, for example, germanium and silicon, but also binary compoundshaving semiconductor properties, for example gallium arsenide. Like inthe case of germanium and silicon, single crystals of a high degree ofpurity are used in the production of binary semiconductors.

The method of producing these single crystals in known manner comprisesseveral steps, that is to say: the reaction of the two elements for theformation of the compound, purification, if necessary, of this compound,doping and production of the single crystal.

The method of purification (by gradual crystallisation of a melt, byzone-melting in an elongated crucible by horizontal displacement or byfloating zone-melting) and the method of producing the single crystal(vertical pulling from a crucible, horizontal zone-melting in anelongated crucible or by floatingzone-melting in a vertical direction)correspond with known methods of producing germanium or silicon singlecrystals.

In the case of binary compounds, however, it is advantageous to startfrom very pure elements prior to the synthesis, so that the phase ofpurification of the compound itself may be dispensed with, so that it iseven possible to carry out simultaneously the synthesis and theformation of the single crystal.

In this case particular precautions have to be taken to ensure that thesemiconductor compound is not contaminated by the atmosphere in thereaction vessel or by the material of the crucible.

The latter possibility of contamination is particularly important inview of the high temperature required for the synthesis whichtemperature favours parasitic reactions with the material of thecrucible and the diffusion of any impurities contained in said material.The risk of contamination by the crucible is the greater, the longer isthe period of contact between the binary, liquid compound at hightemperature and the crucible and the larger is the contact surfacebetween the compound and the crucible.

The reaction between gallium and arsenic for ogtaining gallium'arsenidetakes place at the melting temperature See of gallium arsenide, that isto say at about 1240 C., at which temperature the quartz of the crucibleis slowly reduced by the gallium, so that contamination by silicon isinvolved. Moreover, if the quartz employed is not additionally purified,migration of copper is likely to occur, the copper diffusing rapidly andthen acting as an acceptor. The use of a graphite crucible does notprovide much advantage, since graphite dissolves to a slight extent ingallium.

The influence of these impurities is great, since the formation of agiven quantity of GaAs takes a long period of time. Like several otherbinary IIIV compounds of equal atomic quantities of an element of theGroup III and of an element of the group V of the Periodical System (In,As, Ga, B, P, for example), gallium arsenide has a high dissociationpressure of about 0.9 atm. at the melting temperature. The conventionalmethod of producing gallium arsenide utilizes the action of vapourousarsenic on liquid gallium at a temperature slightly exceeding themelting temperature of gallium arsenide, for example, at 1250 C., whilethe vapour pressure of arsenic is kept at a constant value of 0.9 atm.,that is to say, the dissociation pressure of gallium arsenide at saidtemperature.

With regard to the danger involved in arsenic this reaction is usuallycarried out in a closed quartz ampulla, the colder portion of which isheld at 600 C., at which temperature the vapour pressure of arsenic isequal to the desired pressure in the ampulla, that is to say, 0.9 atm.while the gallium is supplied in a quartz shuttle, which is heated at anappropriate temperature. At 1250 C. the reaction is performed rapidly;the resultant product is a compound of the composition GaAs, which issoluble in gallium at the same temperature. In order to form a body of,for example, 350 gs. it is very likely that, although it is commonpractice to maintain the reaction conditions for one hour, the reactionhas finished already long before.

If during cooling no special precautions are taken the resultant body ispolycrystalline.

It is known that for many uses, particularly in semiconductortechniques, single crystal semiconductor bodies are desired and that asingle crystal is obtained by growth on a seed crystal. This process isperformed gradually and the rate of growth depends mainly upon thetransfer of heat, the uniformity of which determines the quality of theresultant crystal.

The production of single crystals of binary compounds with a markeddissociation pressure at the melting temperature requires, in order toavoid dissociation of the compounds in the molten state, that thereactions should be carried out in an appropriate atmosphere in a closedvessel, the temperature of the colder portion of which usuallydetermines the vapour pressure. In the case of gallium arsenide, forinstance, the vessel employed for the synthesis of the semiconductorcompound may be used, while a crucible is employed which is graduallycooled from one end to the other. In order to obtain satisfactoryresults, this process has to be carried out accurately and for a verylong period of time. A crystal of 350 gs. takes, for example, about 48hours, during which time the liquid gallium arsenide absorbs a growingamount of impurities.

In pulling a single crystal from a melt with the aid of a seed crystalthe quantity solidifying per second depends upon the quantity of heattransferrable per second from the solid substance to the solid liquidinterface and hence also upon the geometric proportions of the systemand upon the pressure, the temperature and the heat exchange with theambience.

During the pulling operation it is preferred to rotate the crystal inorder to avoid asymmetric crystal growth. In order to maintain asatisfactory hermetic closure the vessel in which the crystal is formedis often sealed beforehand, the required control of the moving partsthen being carried out with the aid of magnetic means; it is not.diflicult to obtain in this way a sufficiently uniform movement. Inother known devices a mechanical controlmember is passed through thewall of the vessel by means of a cylindrical, ground through-connection,but in such devices arsenic may leak out, which invcolves the knowndanger and difficulties. Therefore, liquid gallium seals are also usedas an alternative.

All these known devices for crystal pulling have the disadvantage thatduring the Whole process the melt remains in contact with the crucibleat a high temperature.

In floating zone-melting, in which a floating molten zone is held bysurface tension between two aligned solid rod portions of galliumarsenide, a contact between the molten zone and the wall of the crucibleis avoided, but this method requires much care, while during thesynthesis the semiconductor material may have been contaminated.

The present invention has for its object inter alia to mitigate theaforesaid disadvantages.

According to the invention the method of producing a binarymonocrystalline semiconductor compound, on the one'hand, by the reactionof pure, volatile constituents in a closed space, the vapour pressure ofwhich is kept substantially equal to the dissociation pressure of saidcompound at the melting temperature thereof, with a pure moltenconstituent, at a reaction temperature which exceeds said meltingtemperature of the compound, and on the other hand by crystallizing outthe compound in the form of a single crystal by pulling, ischaracterized in that only a zone of the molten, continuouslyreplenished constituent located at the surface near the place where thecrystal is growing during pulling is heated at the reaction temperatureby means of an auxiliary heating member, while the molten compoundobtained by synthesis is directly withdrawn from the melt by the pullingprocess.

The quantity of molten constituent taking part in the reaction, whichmay be contaminated by the high temperature heating, is only small, sothat the method according to the invention provides a compound in theform of a single crystal of a high degree of purity.

The quantity of heat supplied by the auxiliary heating member and therate of pulling are controlled so that the rate of formation of thecompound corresponds to the rate of growth of the crystal.

The level of the molten constituent in the reaction space is preferablyheld constant.

For this purpose the vessel for the melt can be connected by a ducttaken through the wall of the reaction space in a gas-tight manner witha reservoir of the molten constituent, the surface of which is exposedto a chemically controllable pressure of an inert gas, which does notaffect the molten constituent.

The invention will now be described more fully with reference to thedrawing; the figure is a vertical sectional view of apparatus embodyingthe invention for carrying out the method.

Hereinafter the production of single crystals of gallium arsenide ischosen by way of example; as a matter of course, other binarysemiconductor compounds can be obtained by means of the method and thedevice according to the invention.

The device shown in the figure comprises a first space 4, termed thereaction space, which is formed by a hollow, vertical cylinder 1, asecond space 13 and a duct 8 between said spaces.

The duct 8 forms at the same time the horizontal limb of a U-shaped tube26, a vertical limb 30 of which opens out inside the space 4 in the formof a vessel having a widened portion 18 from which the Single crystal is4 pulled up, whereas the other vertical limb 9, which is widened to forma reservoir, opens out inside the space 13.

The open ends of the vertical limbs of the tube 26 are preferablylocated in the one horizontal plane, so that the heights of the levels27 and 28 of the liquids in this tube can be easily observed in thereservoir 9 and in the widened portion 18 through the walls, which aretransparent at least at the relevant areas.

The through-connections 32 and 31 of the duct 8 through the walls of thespaces 4 and 13 are sealed by known means, for example, by melting, ifquartz glass is used.

Through the upper portion of the space 13 is taken 2T slanting tube 10in an airtight manner. The upper wall of the space 13 is provided forthis purpose with a tubular portion 34 for passing the tube 10, while at25 the passage of the tube 10 is sealed in an airtight manner. Insidethe space 13 the tube 10 is prolonged so that at 33 it opens out aboveor in the opening of the reservoir 9. The other end of the tube 10,outside the space 13, is provided with a bulb 12, into which a quantityof gallium 11 can be introduced, which is sufiicient to completely fillthe member 26 after being melted, while it can be continuouslyreplenished during the pulling operation.

The space 13 can be connected through the tube 29 with the outlet duct14 of an exhausting member or with an inlet duct 16 for an inert gas. Bymeans of cocks 15 and 17 the ducts 14 and 16 can be closed wholly orpartly, since the pressure of the inert gas inside the space 13 has tobe variable at will.

The closed vessel 6 is connected at 24 in an airtight manner with thespace 4 through a duct 5 on the side wall of the cylinder 1. The vessel6 is provided with an adequate quantity of crystalline arsenic 7 forobtaining and maintaining in the reaction space 4, subsequent tosublimation, an atmosphere of arsenic of appropriate pressure during thewhole duration of the reaction.

The upper portion of the cylinder 1 has a vertical, cylindrical,narrowed portion 19, the axis of which coincides with the axis of thevertical limb 30 of the U-shaped tube 26 and in which a cylindricalpiston 2 is arranged so as to be vertically displaceable and/orrotatable about its axis. The lower end of the piston is adapted toreceive a monocrystalline seed 3. The lateral surface of the piston 2and the inner wall of the narrowed portion 19 are preferably ground sothat the piston can slide or rotate substantially without friction.However, in spite of a most accurate machining such cylindrical contactsurfaces are not completely gas-tight, which applies particularly tovapourous arsenic. In the device according to the invention the groundseal is improved by a liquid seal formed by an annular trough 20, formedby the cylinder 1 directly above the narrowed portion 19, and containinga liquid of suitable viscosity, for example liquid gallium, if galliumarsenide has to be produced, so that soiling of the outer surface of thepiston 2 and contamination of the atmosphere in the space 4 and hence ofthe growing crystal are avoided.

The length of the ground portion of the piston 2 and the height of thecylinder 1 are sufiicient to allow a vertical translatory movement overa distance corresponding to the length of the single crystal to beproduced.

The vertical translatory movement and the rotary movement of the piston2 can be carried out in known manner by means of a mechanism (notshown), arranged outside the tube 1. The rates of these movements arepreferably variable and uniform. The pulling rate is adapted to theprocess which is the slower in the widened portion 18, that is to say tothe solidification of the gallium arsenic, which is slower than thesynthesis of the gallium arsenide. The solidification is determined bythe heat transfer during the crystallisation and hence by the geometricconditions of the crystal-pulling system.

A furnace 21 provides and maintains the temperature required in thespace 4, the value thereof always exceeding the temperaturecorresponding to the vapour pressure of the arsenic in the vessel 6. Thefurnace 21 surrounds the space 4, the narrowed portion 19 and the partsof the ducts 5 and 8 adjacent the space 4 so that the temperature at anypoint of the space 4 is not lower than the temperature maintained in thevessel 6 for the sublimation of the arsenic. A furnace 22 of known typeprovides and maintains the temperature of the vessel 6 and surrounds thevessel 6 and the portion of the duct 5 extending up to the portionheated by the furnace 21. The temperature of the vessel 6 can bemaintained at a value which is sufficient for evaporating the arsenicuntil the resultant vapour pressure corresponds to the dissociationpressure of gallium arsenide at the melting temperature thereof. Thistemperature of the vessel 6 determines the pressure of the arsenic inthe reaction space.

Moreover, a heating element, which is preferably formed by a coil of asingle winding 23, traversed by a high-frequency current, is capable ofheating the central part of the widened portion 18 of the tube 26 andthe upper portions of the liquid contained therein at a temperaturewhich exceeds that of the remaining part of the space 4, and at whichthe gallium arsenide is liquid. In accordance with the geometricconditions of the system and upon the materials employed it may beadvantageous to provide, above the winding 23, a second winding (notshown) in order to avoid that the heat transfer along the single crystal3 produces an excessively strong cooling of the liquid portions, wherethe compound is being formed. Melting of the charge 11 in the bulb 12 bymeans of a suitable heating element, for example, a simple burner, maybe performed in one step or in several stages. If melting is performedgradually, the bulb 12 is provided internally with a member capable ofdosing the outlet of the molten compound. This member is formed by avertical partition 35, having an opening of a given size on the lowerside or by a different, appropriate member.

The temperatures produced by the furnaces 21 and 22 are controlled andchecked by known means (not shown), formed for example by suitablyarranged thermo-elements and a suitable electric arrangement; especiallythe temperature of the envelope 6 has to be controlled very accurately.Pulling of the single crystal is checked visually through a window inthe furnace 21; this window must not bring about any reduction oftemperature in the space 4.

It is advantageous to add a member for fixing and controlling the level28 of the liquid. This member may be formed by an optical indicatoroperating with a fine light beam or by a detector of passing rays (forexample X- rays) and it permits of keeping said level constant by actingupon the control-member for the pressure of the inert gas in thereservoir 13 with greater accuracy, so that a uniform single crystal isobtained.

The assembly formed by the cylinder 1, the vessels 6 and 12, thereservoir 9, the widened portion 18 and the ducts arranged between themas described above is preferably made of quartz of the purest possiblequality. The through-connections 31, 32, 34 and the connections 24 and25 are preferably welded.

The method according to the invention to be described hereinafter by wayof example and carried out by means of the device shown in the figuresrelates to the production of a single crystal of gallium arsenide.

The gallium arsenide seed crystal 3 is fastened to the lower end of thequartz piston 2, which is inserted into the quartz cylinder 1. Then onecompound, the arsenic, in the crystalline form is put into the quartzvessel 6, which is then hermetically connected at 24 with the quartzduct 5. The required quantity of arsenic depends upon the size of thedesired single crystal; since the gallium arsenide to be produced musthave 50% of the atoms of each constituent, the weight of the arsenic inthe compound is equal to 1.074-times the weight of gallium in thecompound. Above the narrowed portion 19 an adequate quantity of liquidgallium is provided for an effective operation of the seal 20.

The quartz bulb 12 is provided with the gallium charge 11 in the form ofan ingot and the quartz duct 10 is hermetically welded at 25 to saidenvelope. The cock 17 is opened and the cock 15 is closed so that thewhole system can be evacuated at a low temperature for example, for twohours. After degassing the cock 15 is closed. The furnace 21 is switchedon and the gallium 11 is partly heated at the melting temperature ofabout 29 C., for example, by means of a burner until the liquid galliumfills the reservoir 9 through the duct 10, after which it flows throughthe duct 8 into the widened portion 18. The space 4 is thus sealed bythe liquid gallium in the duct 26.

The furnaces 21 and 22 then heat the vessel 6 and the arsenic containedtherein at a temperature of 600 C., which is kept constant with atolerance of 0.25 C. This temperature determines the pressure of thearsenic in the reaction space, which is heated as a whole at a temperature of, for example, 630 C. by the furnace 21. According as thetemperature of the arsenic in the vessel 6 increases, the vapourpressure of the arsenic in the reaction space increases until a value of0.9 atm. is attained at a temperature of 600 C. of the arsenic. In orderto avoid that this pressure urges the liquid gallium from the widenedportion 18 into the reservoir 9, an equal counter-pressure is exerted onthe surface 27 of the liquid gallium in the reservoir 9 by means of aninert gas, for example, argon, which is supplied through the duct 16after the cock 17 is opened. The pressure of the inert gas is controlledby known means at the inlet 16 so that the level 28 of the liquidgallium in the widened portion 18 is located slightly above the heightof the coil 23. Instead of argon nitrogen may be used, since in thedevice according to the invention the gas is in contact with the galliumonly at a temperature which is sufliciently low not to give rise to areaction.

The coil 23 is then connected to a current of, for example, 5 mc./s., sothat the gallium in the widened portion 18 is heated locally at thesurface at a temperature of 1250 C. This temperature is attained only inthe central portion of the liquid surface, since the surroundings areslightly cooled by the wall of the widened portion. The zone heated at1250" C., which value slightly exceeds the melting temperature ofgallium arsenide, is thus restricted to a small surface at the centre ofthe widened portion 18 in line with the piston 2 and the seed 3. Owingto the vapour pressure of the arsenic in the space 4 and to the presenceof the liquid gallium of 1250 C., a small quantity of gallium arsenideis formed at the surface only in a central zone.

The seed crystal 3 is slowly moved downwards until it just touches theliquid surface. The high-frequency current through the coil 23 is againswitched on to melt the end of the seed crystal, so that a drop isformed, which joins the surface of the liquid, at this place by galliumarsenide in statu nascendi.

Pulling of the crystal proper is carried out by slowly and uniformlymoving the piston 2 upwards, the piston being simultaneously rotated inorder to obtain a uniform crystal. The pulling rate is, for example, 2cms./ hour and the speed of rotation is, for example, 20 rev./ min.During the pulling process the liquid gallium is available at thesurface of 1250 C., where locally liquid gallium arsenide is immediatelyformed, so that the risk of contamination is slight, since the galliumarsenide is not at any moment in contact with the quartz or any othermaterial.

The supply of gallium from the widened portion 18 from the reservoir 9via the duct 8 is performed according to need by melting the galliumcharge 11, while a hours for obtaining a single crystal of a length of10 cms. under the aforesaid conditions.

After pulling, the temperatures can be lowered and the whole assemblycan be brought to the ambient temperature, while the gallium of the seal20 is maintained in the liquid state in order to permit of removing thepiston 2 and the single crystal attached thereto.

By the choice of the dimensions of the various parts of the device andof the time of pulling a single crystal of the desired dimensions can beobtained, which has the desired purity, uniformity and homogeneity.

Although in the foregoing the manufacture of single crystals of galliumarsenide is described, the invention is, of course, not restrictedthereto; the device may also be used for the production of singlecrystals of other binary semiconductor compounds, one of theconstituents being supplied in the form of the vapour and the other or acombination of both in the form of a liquid, within the scope of theinvention.

The device according to the invention may also be employed for theproduction of single crystals with impurities or admixtures, supplied inthe vapourous form.

What is claimed is:

1. Apparatus for producing by pulling a monocrystalline binarysemiconductor compound, comprising a closed envelope, within theenvelope means for supporting a supply of a volatile constituent of thecompound, means for heating the supply of the volatile constituent at afirst temperature producing within the envelope a vapor pressure of thevolatile constituent approximately equal to the dissociation pressure ofthe compound at its melting temperature, within the envelope a tubularvessel for holding a molten supply of a non-volatile constituent ofcompound, a duct connected at one end at the bottom of the tubularvessel, a reservoir for the non-volatile constituent connected to theopposite end of the duct, said reservoir being arranged in a closedspace isolated from the said closed envelope, means arranged above thetubular vessel for supporting a seed crystal and for moving the seedcrystal to the melt in the tubular vessel and then pulling the seedcrystal from the melt, heating means for maintaining the space withinthe envelope around the pulling means at a second temperature, heatingmeans for maintaining only the surface of the melt in the vicinity ofthe seed crystal at a third temperature exceeding the meltingtemperature of the compound and at which the melt constituent reactswith the vapor to form the compound which grows on the seed crystalwhile the later is being pulled, means for feeding material in liquidform from the reservoir through the duct into the tubular vessel andthus beneath the melt surface to replenish the melt material lost to thegrowing crystal while avoiding reaction between the said vapor and thereplenishing material before the latter becomes part of the melt supply,and means provided above the reservoir for supporting and melting aningot of the non-volatile constituent and for causing the melt to flowinto the reservoir.

2. Apparatus as set forth in claim 1 wherein the heating means formaintaining the third temperature includes a radio-frequency heatingcoil surrounding the envelope at the level of the melt surface.

3. Apparatus for producing by pulling a monocrystalline binarysemiconductor compound, comprising a closed envelope within the envelopemeans for supporting a supply of a volatile constituent of the compound,means for heating the supply of the volatile constituent at a firsttemperature producing within the envelope a vapor pressure of thevolatile constituent approximately equal to the dissociation pressure ofthe compound at its melting temperature, within the envelope a tubularvessel for holding a molten supply of a non-volatile constituent of thecompound, a duct connected at one end at the bottom of the tubularvessel, a reservoir for the non-volatile constituent connected to theopposite end of the duct, means arranged above the tubular vessel forsupporting a seed crystal and for moving the seed crystal to the melt inthe tubular vessel and then pulling the seed crystal from the melt,heating means for maintaining the space within the envelope around thepulling means at a second temperature, heating means for maintainingonly the surface of the melt in the vicinity of the seed crystal at athird temperature exceeding the melting temperature of the compound andat which the melt constituent reacts with the vapor to form the compoundwhich grows on the seed crystal while the latter is being pulled, meansfor feeding material in liquid form from the reservoir through the ductinto the tubular vessel and thus beneath the melt surface to replenishthe melt material lost to the growing crystal while avoiding reactionbetween the said vapor and the replenishing material before the latterbecomes part of the melt supply, said reservoir being arranged in aclosed space isolated from the said closed envelope, and associated withthe reservoir closed space means for exhausting the closed space andmeans for filling same with an inert gas at a controlled pressure. I

4. Apparatus as set forth in claim 3 wherein the pulling means extendsthrough a wall portion of the envelope sealed off by a melt of thenon-volatile constituent.

References Cited UNITED STATES PATENTS 9/1959 Horn 2330l 2/1963 Enk etal. 23-204 US. Cl. X.R. 23204, 301

